1. Field of the Invention
The present invention relates to a gas sensor element which uses electrodes formed of an electrically conductive oxide and to a gas sensor which includes the gas sensor element.
2. Description of the Related Art
Conventionally, a gas sensor has been known which includes a gas sensor element whose electrical characteristic changes with the concentration of a particular gas component contained in a gas under measurement. For example, Patent Document 1 discloses a gas sensor element which includes a solid electrolyte member having the shape of a bottomed tube closed at its forward end; an inside electrode (a reference electrode) formed on the inner surface of the solid electrolyte member; and an outside electrode (a detection electrode) formed on a forward end portion of the outer surface of the solid electrolyte member. Such a gas sensor is suitably used for detecting the concentration of a particular gas contained in exhaust gas discharged from, for example, a combustor or an internal combustion engine. Also, Patent Documents 2 and 3 disclose various types of electrically conductive oxides. These electrically conductive oxides can be used as an electrode material for a gas sensor element.
Incidentally, with the tightening of emission control standards in recent years, the requirements for gas sensors are becoming more severe year by year. The requirements include (i) higher gas detection accuracy, (ii) excellent heat resistance, and (iii) low cost. When the electrically conductive oxides disclosed in Patent Documents 2 and 3 are used as an electrode material for a gas sensor element, electrodes having a sufficiently low electrical resistance are obtained. Thus, it becomes possible to obtain a gas sensor element which has an improved gas detection accuracy, which is inexpensive as compared with the case where only a noble metal is used as an electrode material, and which has excellent heat resistance.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2009-63330
[Patent Document 2] Japanese Patent No. 3417090
[Patent Document 3] WO 2013/150779
3. Problems to be Solved by the Invention
As to the electrodes of a gas sensor element, not only low electrical and heat resistance, but also resistance to mechanical impact (impact resistance) is required. Each electrode of a gas sensor element has a detection electrode portion used for gas detection and a lead portion for outputting an output signal from the detection electrode portion to an external circuit. In some cases, the lead portion comes into contact with a connection terminal for electrical connection with the external circuit. In such a case, the lead portion must have a sufficiently high impact resistance. Electrically conductive oxide is brittle and has a low impact resistance as compared with noble metal materials. Therefore, when an electrically conductive oxide is merely used as an electrode material for a gas sensor element, sufficiently high impact resistance cannot be attained. Accordingly, a problem arises in that such a gas sensor element and a gas sensor including the same cannot provide the requisite performance.